Obviously, \(a_1b_1\)  should not be zero, which means \(k_1\)  cannot be real or pure imaginary.
This coincides with the assumption on the distribution of eigenvalues from scattering analysis
(cf.[1]}, [2]}).
(REF ) is depicted in Fig.REF (a).
[figure]labelfont=bf,name=Fig.,labelsep=period
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